The present invention relates to a luminescence device and a metal coordination compound therefor. More specifically, the present invention relates to a luminescence device employing an organic metal coordination compound having a platinum center metal as a luminescence material so as to allow stable luminescence efficiency, and a metal coordination compound adapted for use in the luminescence device.
An organic electroluminescence EL device has been extensively studied as a luminescence device with a high responsiveness and high efficiency.
The organic EL device generally has a sectional structure as shown in FIG. 1A or 1B (e.g., as described in xe2x80x9cMacromol. Symp.xe2x80x9d, 125, pp. 1-48 (1997)).
Referring to the figures, the EL device generally has a structure including a transparent substrate 15, a transparent electrode 14 disposed on the transparent substrate 15, a metal electrode 11 disposed opposite to the transparent electrode 14, and a plurality of organic (compound) layers disposed between the transparent electrode 14 and the metal electrode 11.
Referring to FIG. 1, the EL device in this embodiment has two organic layers including a luminescence layer 12 and a hole transport layer 13.
The transparent electrode 14 may be formed of a film of ITO (indium tin oxide) having a larger work function to ensure a good hole injection performance into the hole transport layer. On the other hand, the metal electrode 11 may be formed of a layer of aluminum, magnesium, alloys thereof, etc., having a smaller work function to ensure a good electron injection performance into the organic layer(s).
These (transparent and metal) electrodes 14 and 11 may be formed in a thickness of 50-200 nm.
The luminescence layer 12 may be formed of, e.g., an aluminum quinolinol complex (representative example thereof may include Alq3 described hereinafter) having an electron transporting characteristic and a luminescent characteristic. The hole transport layer 13 may be formed of, e.g., a triphenyldiamine derivative (representative example thereof may include xcex1-NPD described hereinafter) having an electron donating characteristic.
The above-described EL device exhibits a rectification characteristic, so that when an electric field is applied between the metal electrode 11 as a cathode and the transparent electrode 14 as an anode, electrons are injected from the metal electrode 11 into the luminescence layer 12 and holes are injected from the transparent electrodes 14.
The thus-injected holes and electrons are recombined within the luminescence layer 12 to produce excitons, thus causing luminescence. At that time, the hole transport layer 13 functions as an electron-blocking layer to increase a recombination efficiency at the boundary between the luminescence layer 12 and the hole transport layer 13, thus enhancing a luminescence efficiency.
Referring to FIG. 1B, in addition to the layers shown in FIG. 1A, an electron transport layer 16 is disposed between the metal electrode 11 and the luminescence layer 12, whereby an effective carrier blocking performance can be ensured by separating the functions of luminescence, electron transport and hole transport, thus allowing effective luminescence.
The electron transport layer 16 may be formed of, e.g., oxadiazole derivatives.
In ordinary organic EL devices, fluorescence caused during a transition of a luminescent center molecule from a singlet excited state to a ground state is used as luminescence.
On the other hand, not the above fluorescence (luminescence) via singlet exciton, but phosphorescence (luminescence) via a triplet exciton has been studied for use in an organic EL device as described in, e.g., xe2x80x9cImproved energy transfer in electrophosphorescent devicexe2x80x9d (D. F. O""Brien et al., Applied Physics Letters, Vol. 74, No. 3, pp. 442-444 (1999)) and xe2x80x9cVery high-efficiency green organic light-emitting devices based on electrophosphorescencexe2x80x9d (M. A. Baldo et al., Applied Physics Letters, Vol. 75, No. 1, pp. 4-6(1999)).
The EL devices shown in these documents may generally have a sectional structure shown in FIG. 1C.
Referring to FIG. 1C, four organic layers including a hole transfer layer 13, a luminescence layer 12, an exciton diffusion-prevention layer 17, and an electron transport layer 16 are successively formed in this order on the transparent electrode (anode) 14.
In the above documents, higher efficiencies have been achieved by using four organic layers including a hole transport layer 13 of xcex1-NPD (shown below), an electron transport layer 16 of Alq3 (shown below), an exciton diffusion-prevention layer 17 of BPC (shown below), and a luminescence layer 12 of a mixture of CPB (shown below) as a host material with Ir(ppy)3 (shown below) or PtOEP (shown below) as a guest phosphorescence material doped into CBP at a concentration of ca. 6 wt. %. 
Alq3: tris(8-hydroxyquinoline) aluminum (aluminum-quinolinol complex),
xcex1-NPD: N4,N4xe2x80x2-di-naphthalene-1-yl-N4,N4xe2x80x2-diphenyl-biphenyl-4,4xe2x80x2-diamine (4,4xe2x80x2-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl),
CBP: 4,4-N,Nxe2x80x2-dicarbazole-biphenyl,
BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline,
Ir(ppy)3: fac tris(2-phenylpyridine)iridium (iridium-phenylpyridine complex), and
PtEOP: 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (platinum-octaethyl porphine complex).
The phosphorescence (luminescence) material used in the luminescence layer 12 has attracted notice. This is because the phosphorescence material is expected to provide a higher luminescence efficiency in principle.
More specifically, in the case of the phosphorescence material, excitons produced by a recombination of carriers comprise singlet excitons and triplet excitons presented in a ratio of 1:3. For this reason, when fluorescence caused during the transition from the singlet excited state to the ground state is utilized, a resultant luminescence efficiency is 25% (as upper limit) based on all the produced excitons in principle.
On the other hand, in the case of utilizing phosphorescence caused during a transition from the triplet excited state, a resultant luminescence efficiency is expected to be at least three times that of the case of fluorescence in principle. In addition thereto, if an intersystem crossing from the singlet excited state (higher energy level) to the triplet excited state is taken into consideration, the luminescence efficiency of phosphorescence can be expected to be 100% (four times that of fluorescence) in principle.
The use of phosphorescence based on transition from the triplet excited state has also been proposed in, e.g., Japanese Laid-Open Patent Application (JP-A) 11-329739, JP-A 11-256148 and JP-A 8-319482.
However, the above-mentioned organic EL devices utilizing phosphorescence have a problem associated with luminescent deterioration particularly in an energized state.
The reason for luminescent deterioration has not been clarified as yet but may be attributable to such a phenomenon that the life of triplet exciton is generally longer than that of singlet exciton by at least three digits, so that a molecule is placed in a higher-energy state for a long period, causing a reaction with an ambient substance, formation of exciplex or excimer, a change in a minute molecular structure, a structural change of an ambient substance, etc.
Accordingly, the (electro)phosphorescence EL device is expected to provide a higher luminescence efficiency as described above, while the EL device is required to suppress or minimize the luminescent deterioration in the energized state.
An object of the present invention is to provide a luminescence device capable of providing a high-efficiency luminescent state at a high brightness (or luminance) for a long period while minimizing the deterioration in luminescence in the energized state.
Another object of the present invention is to provide a metal coordination compound as a material suitable for an organic layer for the luminescence device.
According to the present invention, there is provided a luminescence device, comprising: an organic compound layer comprising a metal coordination compound having a partial structure represented by the following formula (1): 
wherein each of N and C represents an atom constituting a cyclic group.
According to the present invention, there is also provided a metal coordination compound, adapted for use in a luminescence device, having a partial structure represented by the following formula (1): 
wherein each of N and C represents an atom constituting a cyclic group.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawing.